Combustion heaters of conventional heating systems, also called furnaces, often employ tubular combustion chambers, also called heat exchangers, wherein a mixture of gaseous fuel and air is burned, and the combustion products resulting from the burning are directed to a flue and ultimately to an exhaust. Air to be conditioned is usually returned from a living/working space and passed over the tubular combustion chambers, where it takes on heat from the combustion chambers and then is routed back to a living or working space. As a result of the combustion process, combustion systems normally generate gaseous combustion products, including NOx, which are vented to the atmosphere as flue gas. It is desirable to limit these NOx emissions since NOx is considered a pollutant, and combustion systems sold in certain jurisdictions must meet strict NOx emission standards.
One technique for limiting NOx emissions from a combustion system is to control peak combustion flame temperatures that contact the tubular combustion chambers as well as limiting the residence times at the peak temperatures to minimize the formation of NOx. It has been known that peak combustion flame temperatures can be controlled by locating a flame holder, also called a baffle, into the combustion tube to contain the flame at least partially and discourage it from directly contacting the combustion tube. Baffles have been in wide use for many years in commercial and residential furnaces.